Thick-crusted algae living on the seafloor of the Arctic Ocean hold 650 years' worth of data on sea-ice cover.

A new study analyzing the crusty records of the Arctic algae Clathromorphum compactumshows that sea-ice coverage in the Arctic, North Atlantic and North Pacific has been declining steadily since 1850. The underwater record also reveals that the 20th century had the lowest sea-ice cover of the last 646 years, researchers report November 18 in the Proceedings of the National Academy of Sciences.

The algal crusts record information about yearly sea-ice coverage much like trees' rings preserve data about annual precipitation. Arctic algae's sea-ice records could offer scientists a new tool to reconstruct climates much farther back in history, which could help to improve climate models of the future, the scientists suggest.